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                外太空旅行                </h1>
                <p>时间限制：5s&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  空间限制：128MB</p>			</div>
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                                <h3>题目描述</h3><p><div style="margin: 6pt 0cm 0pt; text-indent: 21pt"><span style="font-size: medium">在人类的触角伸向银河系的边缘之际，普通人上太空旅行已经变得稀松平常了。某理科试验班有n个人，现在班主任要从中选出尽量多的人去参加一次太空旅行活动。</span></div>
<div style="margin: 6pt 0cm 0pt; text-indent: 21pt"><span style="font-size: medium">可是n名同学并不是和平相处的。有的人，比如小A和小B整天狼狈为奸，是好朋友；但还有的人，比如杜鲁门和赫鲁晓夫就水火不相容。这n名同学，由于是理科生，都非常的理性，所以&ldquo;朋友的朋友就是朋友&rdquo;和&ldquo;敌人的朋友就是敌人&rdquo;这两句话对这些同学无效。换句话说，有可能小A和小B是朋友，小B和小C是朋友，但是小A和小C两人势如水火。</span></div>
<div style="margin: 6pt 0cm 0pt; text-indent: 21pt"><span style="font-size: medium">任意两个人之间要不就是敌人，要不就是朋友。</span></div>
<div style="margin: 6pt 0cm 0pt; text-indent: 21pt"><span style="font-size: medium">因为在太空船上发生人员斗殴事件是很恶劣也很危险的，因此选出来参加旅行活动的同学必须互相之间都是朋友。你的任务就是确定最多可以选多少人参加旅行。</span></div>
<div style="margin: 6pt 0cm 0pt"><span style="font-size: medium">&nbsp;</span></div></p><hr/><h3>输入格式</h3><p><div style="margin: 6pt 0cm 0pt; text-indent: 21pt"><span style="font-size: medium">第一行一个整数n(1&lt;=n&lt;=50)。所有的同学按照1~n编号。</span></div>
<div style="margin: 6pt 0cm 0pt; text-indent: 21pt"><span style="font-size: medium">接下来若干行，每行两个用空格隔开的整数a, b(1&lt;=a,b&lt;=n)，表示a和b是朋友。</span></div>
<div style="margin: 6pt 0cm 0pt; text-indent: 21pt"><span style="font-size: medium">注意：如果一个数对(x,y)（或者(y,x)）没有在文件中出现，那么编号为x和y的两个同学就是敌人。</span></div>
<div style="margin: 6pt 0cm 0pt"><span style="font-size: medium">&nbsp;</span></div></p><hr/><h3>输出格式</h3><p><div style="margin: 6pt 0cm 0pt; text-indent: 21pt"><span style="font-size: medium">仅仅一个数，即最多可以选多少人参加活动。</span></div>
<div style="margin: 6pt 0cm 0pt"><span style="font-size: medium">&nbsp;</span></div></p><hr/><h3>样例输入</h3><pre>4
1 2
2 3
3 1
1 4
</pre><hr/><h3>样例输出</h3><pre>3
说明：选编号为1,2,3的同学参加，他们互相都是朋友。</pre><hr/><h3>提示</h3><p>没有写明提示</p><hr/><h3>题目来源</h3><p>没有写明来源</p>
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